Model Predictive control of spacecraft

Abstract

A method controls an operation of a spacecraft according to a model of the spacecraft. The method determines control inputs for controlling concurrently thrusters of the spacecraft and momentum exchange devices of the spacecraft using an optimization of a cost function over a receding horizon subject to constraints on a pose of the spacecraft and constraints on inputs to the thrusters. The cost function includes components for controlling the pose of the spacecraft and a momentum stored by the momentum exchange devices. The method generates a command to control concurrently the thrusters and the momentum exchange devices according to at least a portion of the control inputs.

Claimed is: 
     
       1. A method for controlling an operation of a spacecraft according to a model of the spacecraft, comprising:
 determining control inputs for controlling concurrently thrusters of the spacecraft and momentum exchange devices of the spacecraft using an optimization of a cost function over a receding horizon subject to constraints on a pose of the spacecraft and constraints on inputs to the thrusters, wherein the cost function includes components for controlling the pose of the spacecraft and a momentum stored by the momentum exchange devices including a component for a position of the spacecraft penalizing a displacement of the spacecraft from a desired position and a component for the stored momentum penalizing larger value of a magnitude of the stored momentum; and 
 generating a command to control concurrently the thrusters and the momentum exchange devices according to at least a portion of the control inputs, wherein steps of the method are performed by a processor of the spacecraft. 
 
     
     
       2. The method of  claim 1 , wherein the optimization is based on the model of the spacecraft including a nominal model defining relationships among parameters of the model and a disturbance model defining disturbance forces acting on the spacecraft. 
     
     
       3. The method of  claim 2 , further comprising:
 performing a linearization of the nominal model as if the spacecraft is located at a target position for the entire period of the receding horizon; and 
 determining the disturbance forces as if the spacecraft is located at the target position for the entire period of the receding horizon. 
 
     
     
       4. The method of  claim 1 , wherein the constraints on the pose of the spacecraft include a position constraint maintaining a position of the spacecraft within a predetermined window and an orientation constraint maintaining Euler Angles of the spacecraft within a predetermined limit. 
     
     
       5. The method of  claim 1 , wherein the constraints on the inputs to the thrusters guarantees an ability of the thrusters to jointly generate a force for controlling the pose of the spacecraft and a torque for unloading the momentum stored by the momentum exchange devices of the spacecraft. 
     
     
       6. The method of  claim 1 , further comprising:
 weighting each of the components of the cost function, such that the optimization of the cost function produces control inputs that achieve goals of each individual component with priority corresponding to their relative weight. 
 
     
     
       7. The method of  claim 6 , wherein the control inputs are determined iteratively, and wherein at least one iteration comprises:
 updating one or combination of the components of the cost function and weights of the components of the cost function based on a change of a desired operation of the spacecraft. 
 
     
     
       8. A control system for controlling an operation of a spacecraft according to a model of the spacecraft, comprising at least one processor for executing modules of the control system, the modules comprising:
 a control input module for determining control inputs for controlling concurrently thrusters of the spacecraft and momentum exchange devices of the spacecraft using an optimization of a cost function over a receding horizon subject to constraints on a pose of the spacecraft and constraints on inputs to the thrusters, wherein the cost function includes components for controlling the pose of the spacecraft and a momentum stored by the momentum exchange devices including a component for a position of the spacecraft penalizing a displacement of the spacecraft from a desired position and a component for the stored momentum penalizing larger value of a magnitude of the stored momentum; and 
 a force-torque map module for generating a command to control concurrently the thrusters and the momentum exchange devices according to at least a portion of the control inputs, wherein the generated command includes a command to the momentum exchange devices to unload the stored momentum and commands to individual thrusters to generate forces and torques to maintain or change the pose of the spacecraft and to compensate for a torque generated by the momentum exchange devices unloading the stored momentum. 
 
     
     
       9. The control system of  claim 8 , wherein the optimization is based on the model of the spacecraft including a nominal model defining relationships among parameters of the model and a disturbance model defining disturbance forces acting on the spacecraft, further comprising:
 a current model module for linearizing the nominal model and determining the disturbance forces as if the spacecraft is located at a target position for the entire period of the receding horizon. 
 
     
     
       10. The control system of  claim 8 , wherein the constraints on the pose of the spacecraft include a position constraint maintaining a position of the spacecraft within a predetermined window and an orientation constraint maintaining Euler Angles of the spacecraft within a predetermined limit, and wherein the constraints on the inputs to the thrusters guarantees an ability of the thrusters to jointly generate a force for controlling the pose of the spacecraft and a torque for unloading the momentum stored by the momentum exchange devices of the spacecraft. 
     
     
       11. A spacecraft comprising:
 a set of thrusters for changing a pose of the spacecraft; 
 a set of momentum exchange devices for absorbing disturbance torques acting on the spacecraft; and 
 the control system of  claim 8  for controlling the thrusters and the momentum exchange devices. 
 
     
     
       12. A spacecraft comprising:
 a set of thrusters for changing a pose of the spacecraft; 
 a set of momentum exchange devices for absorbing disturbance torques acting on the spacecraft; and 
 a control system for controlling concurrently operations of the thrusters and the momentum exchange devices, the control system includes at least one processor for executing modules of the control system, the modules comprising:
 a control input module for determining control inputs for controlling concurrently thrusters of the spacecraft and momentum exchange devices of the spacecraft using an optimization of a cost function over a receding horizon subject to constraints on a pose of the spacecraft and constraints on inputs to the thrusters, wherein the cost function includes components for controlling the pose of the spacecraft and a momentum stored by the momentum exchange devices including a component for a position of the spacecraft penalizing a displacement of the spacecraft from a desired position and a component for the stored momentum penalizing larger value of a magnitude of the stored momentum; and 
 a force-torque map module for generating a command to control the thrusters according to at least a portion of the control inputs to generate forces and torques to maintain or change the pose of the spacecraft and to compensate for a torque generated by the momentum exchange devices unloading the stored momentum.